This invention relates to an improvement in a rotary-type vacuum pump which has means for preventing contamination caused by a backward flow of oil and steam in the pump unit when the operation of the pump unit is stopped by unexpected power failure or breakdown.
In a rotary-type vacuum pump, oil is used as a sealing means for obtaining a high degree of vacuum. Also, the oil cools the pump unit and lubricates the bearings and other parts while circulating through the pump unit.
For forced circulation of the oil through the pump unit, an auxiliary lubricating pump or oil pump may be provided. When the operation of the pump unit or the supply of oil is suddenly stopped, oil and air in the pump or the oil storage casing are likely to flow backward into the pump unit due to the decrease of pressure. Thereafter, the backward flowing oil and air enter the vacuum equipment through the suction duct, thus contaminating and seriously damaging the vacuum equipment. Accordingly, a non-return valve for closing the suction duct and non-return valve control means should be provided.
A conventional non-return valve control means opens the non-return valve only when the fluid pressure through the lubrication circuit is restored. This presents a problem in that substantial time is needed for pressure restoration, and for the pump unit to re-operate.
U.S. Pat. No. 4,844,702 issued Jul. 4, 1989 discloses a solution to this problem.
This U.S. Patent relates to an improved lubrication circuit which is connected to rotary vacuum pumps and is composed of a hydraulic circuit and of the related pump unit for lubrication and auxiliary controls, including isolation of the negative pressure space from the pump upon the stopping of the latter, with the aid of a closure member.
In the lubrication circuit, the pressure of the fluid in pump discharge space is used for controlling a closure member, with the aid of two ducts connected with each other between the discharge space and a closure member operating member. The hydraulic pressure generated by lubrication pump unit is applied to the moving parts of the pump and transmitted through the pressure transmitting duct to a control member which controls the operation of the closure member.
However, the U.S. Patent has the following problems.
In the first place, the oil entry port for entry of the oil from the pump (or oil storage casing) to the pump unit is disposed high. Therefore, a slight lowering of the oil surface will cause stoppage of the oil supply. This causes non-operation of the closure member.
Secondly, since the filter net is disposed in the oil entry of the oil pump, if the filter net is clogged, substantial time is needed for repair it.
Thirdly, excess oil should be returned through the excess pressure discharge valve. However, in the U.S. patent, since oil pump and pressure discharge valve are adjacent to each other, the oil to be supplied to the lubrication circuit is likely to leak through the pressure discharge valve before being supplied to the moving parts.
Fourthly, when returned to the oil storage casing through the pressure discharge valve, oil will be sprayed by high hydraulic pressure and hit the inner face of case, thus producing noise and being discharged through the discharge port together with discharged gases. This causes waste of oil.
Fifthly, the spring mounted between vanes for making the vanes come in close contact with the inner face of the cylinder is likely to break easily due to repetitive tension and compression stress. This causes substantial cost for operating the pump unit.